Implantable medical devices may be employed in various applications. For example, an implantable cardiac device may perform one or more functions including sensing signals generated in a patient's heart, pacing the heart to maintain regular contractions, and providing defibrillation shocks to the heart. Similarly, an implantable stimulation device may be used to apply stimulation signals to a patients muscular tissue, neurological system, or some other area of the patient's body.
In practice, there may be a need to communicate with an implantable medical device after it has been implanted in a patient. For example, an external monitoring device located in a person's home, a doctor's office, a clinic, or some other suitable location may be used to retrieve information collected by and/or stored in the implanted medical device. In the case of an implanted cardiac device, such information may include sensed cardiac activity data that a treating physician may analyze to learn about the patient's health. Similarly, an external programming device located in any of the above locations may be used by a treating physician to change the operating parameters of the implanted medical device. Such parameters may include, for example, the timing or magnitude of stimulation pulses generated by the implanted medical device.
In a typical implementation, an implanted medical device utilizes radio frequency (“RF”) telemetry to communicate with an external device. Consequently, the implanted medical device may include an RF transceiver that is adapted to transmit and receive RF signals. In such an implementation, however, it is generally desirable to leave the transceiver in a powered-off or low power state as much as possible since the transceiver may consume a relatively large amount of power. Here, it should be appreciated that the replacement of the battery in an implanted medical device involves a surgical procedure. Hence, long battery life is an important aspect of such a device.
Some types of implanted medical devices employ a wakeup scheme whereby an implanted device will periodically turn on its transceiver (e.g., its receiver) to determine whether an external device is attempting to establish a communication session. For example, whenever an external device wishes to establish communication with an implanted medical device, the external device may periodically transmit polling messages (e.g., connection requests) over one or more designated RF channels. Each of these polling messages may include information relating to establishing the communication such as, for example, an identifier that uniquely identifies the implanted medical device.
Every time the transceiver of the implanted medical device is turned on (e.g., at defined intervals), the transceiver may conduct an RF scan to determine whether the external device is transmitting polling messages. This may involve, for example, performing an ID scan that checks each RF channel for any messages that include an identifier associated with that particular implanted medical device. In the event such a message is detected, the implanted medical device transmits one or more signals (e.g., in accordance with a handshake protocol) to establish communication with the external device.
In practice, a transceiver of an implanted medical device may consume a relatively significant amount of power even when a conventional wakeup scheme is used. For example, it may be desirable for an implanted medical device to be able to respond to polling messages within a relatively short period of time, for example, so that a treating physician need not wait several minutes to establish communication with the implanted medical device. The implanted medical device may therefore perform its scans at relatively frequent intervals to achieve a quick response time. Such frequent scanning may, however, increase the amount of power consumed by the implanted medical device.
In some cases, a wakeup scheme may involve staged detection to reduce the amount of power consumed by the implanted medical device. For example, such a scheme may employ a simple detection stage that samples RF energy and a more robust scanning stage that analyzes any detected signals to determine whether these signals are from an external device that is attempting to establish communication with the implanted medical device. Here, the simple detection stage may repeatedly perform an energy “sniff” in the RF channel or channels of interest. By starting a detection search with this low-level and relatively coarse energy assessment stage, the implanted medical device may avoid using the relatively high power robust scanning stage during those times when the external device is not transmitting a signal which, in practice, is usually the case for most of the lifetime of the implanted medical device. Hence, additional power savings may be achieved with this type of wakeup scheme in comparison to a wakeup scheme that does not employ staged detection.